Filtering AC from DC circuit

Discussion in 'General Electronics Chat' started by badmonkey91, Oct 6, 2005.

  1. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    I have an AC adapter that puts out 5VDC and 15vac, it hooks up to a circuit that needs to convert it to 3VDC, but it puts out 5vac too.

    I was wondering if this would damage the gameboy advance it will power, and if so, how would I filter the ac.

    I tried a cap directly from positive to negative, it seemed to work, but then stopped. Did something randomly short or was I wrong to think that would work? Is there anything else I could do to stop it from getting to the end of the circuit without stopping the DC?
     
  2. tronicsguy

    New Member

    Oct 6, 2005
    9
    0
    Is it a AC riding on 5V DC?
    Putting a capacitor is the right way to go. Make sure your cap is rated at 30V or higher. Atleast twice the peak to peak AC amplitude.
    The cap should bypass the AC and allow the DC.
    Try using a large value cap. start with 10 micro farad.
    A choke in series also helps.
     
  3. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    whats a choke? diodes?
     
  4. Sebi

    Well-Known Member

    Sep 24, 2005
    59
    0
    Inductor (1...5H serial)
     
  5. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    I finished, I dont think it works. The AC still shows up in the output, but only one way. I really dont know x_x
     
  6. tronicsguy

    New Member

    Oct 6, 2005
    9
    0
    shows up only one way? can u explain?
    what was the capacitor value you put?
    Do you have the adapter model #?
     
  7. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    I dunno about it showing up one way, I put the voltmeter's leads on the output the correct way, the ac shows up. If I reverse the leads (the oppisite way[that the DC runs])it doesnt show up. I do have a diode in there.

    The cap is a 22μf (microfarad), and there is also a coil in series.

    The adapter is a model SY-03300 Class 2 Power supply manufactured by the Sha Yao Eectric Factory (LISTED e204894 35xj [unsure what that is]). Input 120vac, 60Hzm 50mA, Output 3VDC 300mA (it does not measure this, it puts out about 5-8VDC and 15vac)

    [​IMG]
     
  8. Overclocked2300

    Senior Member

    Apr 24, 2005
    124
    0
    Put the diode before the cap, so that it will rectifie the AC and pass the DC.Keep everything else.

    You may also need a bigger cap.
     
  9. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    I changed the 22 μf cap to 220 μf, and put the diode infront of the cap. Still getting the ac in the output. ~:( there's 2 hours wasted <_< I have a really small area to work with, and limited parts. x_x

    [​IMG]
    gah! I dont get it, this should work >.<

    should I put the cap after the resister too? hmm..
     
  10. Overclocked2300

    Senior Member

    Apr 24, 2005
    124
    0
    Put the diode on top(pos side), not on the negative side

    I just relized your polarties are reversed. Top is pos, bottom is neg
     
  11. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    what? no, I don't think my polarities are backwards, here, I made a schematic with arrows representing the electron flow.

    [​IMG]

    we're not talkin hole flow here :p

    I'm glad for the attempts to help, please state some proof to back your ideas, as I actually have to pyshically change my design everytime. o_o

    suppose I'll break out the ol' breadboard
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Which AC source do you want to be the source for your 3 VDC supply or do you wish to use the 5 volt DC?
     
  13. badmonkey91

    Thread Starter New Member

    Oct 6, 2005
    7
    0
    I just want to eliminate the ac.
     
  14. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    i will presume that you are measuring the AC with your multimeter in the AC measurement mode that's why you are getting that reading, right?

    could someone pls. explain why you would still get an AC reading if your multimeter is in AC measurement mode inspite that you are checking the DC output?

    moz
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    If your DVM can handle the frequencies, you'll be reading the ripple voltage on a DC supply when the DVM is in the AC positioin.

    I've seen power supplies with 5 mV ripple as the specification on a 24 V DC line. It's tough to measure with an oscilloscope [years ago] using a times 10 probe and the lowest setting on the scope was 5 mV. That meant the specification was one half of one small vertical division ... or slightly above the trace width with the intensity barely visible and the lights in the room off.
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Do you have a picture of this supply?

    I'm having a difficult time understanding your " Input 120vac, 60Hzm 50mA, Output 3VDC 300mA (it does not measure this, it puts out about 5-8VDC and 15vac)" statement.

    Are you saying when using the AC mode on your meter, your reading 15 VAC, and at the same two measuring points your reading 5-8 VDC when in the DC mode? I find it difficult to believe there is 15 VAC ripple on an 8 VDC line without there being a defect in the power supply.

    Do you have access to an oscilloscope?
     
  17. tronicsguy

    New Member

    Oct 6, 2005
    9
    0
    badmonkey, plz get hold of an oscilloscope and try to look at the output waveforms. which multimeter are u uisng.
    To identify +ve and -ve, put your DVM in DC voltage mode and put the Red test probe on the outer part of the adapter pin and the black part on the inner part. If the DVM shows the voltage with a -ve sign, then the polarity is opposite. otherwise, the outer part will be +ve and inner -ve.
    you might have done this already. In that case just ignore.



    Moz,
    If your DVM is in AC mode and measuring DC, it will give you the rms value of the voltage thinking its AC.
    The DVM averages if its in DC mode. Average of AC is zero.
    In AC, it gives the rms value which is sqrt (2)*peak value.
     
Loading...